1. Field of the Invention
The present invention generally relates to substrates which interconnect and dissipate heat from power semiconductor elements and more particularly to integrated substrates having enhanced thermal characteristics in which interconnecting wiring patterns for power and control circuit elements are combined.
2. Description of Related Art
Industry has been continually increasing the demand for higher speeds, smaller size, and increased power from circuit boards. Such design requirements are difficult to achieve with the concomitant requirement of removing the heat generated by high density high power designs.
The conventional approach to high density high power design has been the use of power hybrid microcircuits, which include an alumina ceramic substrate base onto which a current-carrying interconnect pattern is either deposited or screen printed. Unfortunately, this interconnect is not suitable for the high current generated by "power semiconductor elements." Instead, "bonding islands" are solder mounted onto the interconnect and in turn, wires that are capable of carrying high current or power semiconductors are mounted onto the islands. The resulting assembly may require a hermetically packaged enclosure. Low current control circuitry, such as integrated circuits, transistors, resistors and capacitors, which serve as control circuits for the power semiconductors, are generally built on a separate substrate in the same enclosure or in a completely separate module.
One recent prior art approach to power modules involves chemically bonding a thin copper layer to an alumina (aluminum oxide) or aluminum nitride ceramic substrate using an oxide interface. The thin copper layer is then electroplated to increase its thickness to greater than 0.005 inches. The substrate is then used to interconnect high current devices. However, current carrying capability is a direct function of cross-sectional area and a high current device therefore requires either a wider or a thicker conductor to have sufficient cross-sectional area. The drawback with alumina or aluminum nitride ceramic substrates which use 0.005 inch thick copper is that these modules are not able to carry high currents without requiring a larger surface area on the module, thereby unsuitably increasing the overall size of the module.
Further developments in the area of alumina or aluminum nitride ceramic substrate technology include chemically bonding thicker copper foils, greater than 0.005 inches, to a ceramic substrate. Even with a thicker foil, however, ceramic based substrates have drawbacks. A ceramic based substrate generally must be mounted on a structural base, otherwise the brittle ceramic material can break. In most applications, size of the circuit is an important consideration, and it is undesirable to waste valuable space with the additional structural base required to support the ceramic substrate. Furthermore, even in applications where a structural base can be avoided, the brittle ceramic substrates require special fasteners to avoid breakage.
Using metal instead of a ceramic base is known in the art. For example, the design disclosed in U.S. Pat. No. 5,513,072 involves a common module containing a power semiconductor element, a circuit board for the power element, a circuit board for a control circuit and a heat spreader fixed on a metal plate. In this design, the power element is mounted onto the heat spreader, which is formed as a copper or copper clad island. In turn, the heat spreader is adhered to the metal plate by a high heat radiating insulating layer made of resin based material so that the heat spreader and the metal plate are electrically insulated from each other. The wiring pattern for the power semiconductor element is formed in a power circuit control board disposed adjacent to one side of the power element on the metal plate. Attached to the metal plate and placed on the opposite side of the power semiconductor is an additional circuit board which is used for wiring control circuitry.
A power module as disclosed by U.S. Pat. No. 5,513,072 includes high and low power devices on one base plate, but suffers drawbacks. First, the wiring pattern for the power semiconductor cannot be placed beneath the power semiconductor in such a design. Instead, a power circuit board is placed adjacent to the power element, thereby using more valuable space on the module. A design allowing the power element wiring pattern to be placed intermediate the power semiconductor element and the metal plate would save valuable space and achieve a smaller module. Furthermore, the design requires a separate heat spreader positioned between the power element and the metal base in order to dissipate heat. A thinner, cheaper module could be constructed if the heat spreader could be eliminated. Finally, a power module like that disclosed by U.S. Pat. No. 5,513,072 requires two separate circuit boards, one of which is for interconnecting high power elements while the other circuit board connects the low power circuitry. A power module having two instead of one circuit board adds a step to the manufacturing process, in turn increasing cost. Furthermore, a power module having two circuit boards requires a larger design than a power module with one circuit board.
It has been known to use thick copper foils in conjunction with metal substrates such as aluminum to remove heat generated by power semiconductors. One known device is an automotive power steering module that utilizes a large (8 inches by 4 inches) copper interconnect pattern bonded to an aluminum substrate. The interconnect pattern is approximately 0.020 inch thick and interconnects large power semiconductors and switches currents greater than 75 amperes. In such an application, the aluminum base serves not only as the mounting platform for the semiconductors but is also an integral part of the system, serving as a structural mounting member in the power steering assembly. In this known application, a low power interconnect using a single layer printed circuit board is bonded to the thick copper foil and provides control signal connections.
Another known product is a power module for a surgical saw. In this power module, a copper interconnect pattern having a thickness greater than 0.010 inches is again bonded to an aluminum substrate and is used to interconnect the switching power transistors that control the brushless motor in the surgical saw. The saw features a high torque which requires switching of currents greater than 30 amperes by the power module.
Yet another known prior art product is a power module for an automotive anti-lock braking system ("ABS system"). This known module also utilizes a thick copper interconnect pattern bonded to an aluminum substrate to interconnect power transistors and to switch the large currents required in the ABS system. As in the above described power steering module, the module for the automotive ABS system also is formed as an integral part of the system, thereby giving structural strength to the overall assembly.
Although some developments have been made in integrating power and control circuit elements while removing heat, it is desirable to further improve the interconnect density and the heat removal capacity of such substrates. It is thus desirable to have a single substrate that is able to (1) handle high currents generated by power semiconductors; (2) route many interconnections in a small area; (3) efficiently remove heat at a reduced cost; (4) integrate low power circuitry required to control high current devices; and (5) offer an overall reduction in size. In addition to these performance features, it is desirable to have an integrated substrate that lends itself to mass production at low cost.